I'm wondering why is the single leg version of the deadlift is usually performed with stiff legs. For instance, exrx.net and most of the youtube videos that teach the single leg deadlift show it with stiff legs. What is the reason for this? Are there any risks when performing the single leg deadlift like the standard one?

2 Answers
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A big portion of it is simply the mechanics of the lift. Just try to stand on one leg in a conventional deadlift stance (sumo would be impossible). You run into the big question of what to do with the leg you aren't using. If you stick it in front of you like a pistol, grabbing the bar becomes nearly impossible. If you stick it behind you there is a high likelihood that you don't have the flexibility to keep it behind you like a half split while you initiate the pull.

The stiff single leg deadlift is a natural movement

You have the advantage of the other leg acting as a cantilever to help you lift heavier

The stiff leg helps target the hamstrings and glutes more, which make it a good assistance exercise.

The second portion is understanding what the exercise is designed to target. The primary part of the posterior chain that single leg deadlifts are hitting is the hamstrings, and secondarily the glutes. As such, you need to follow these guidelines:

Don't lock your knee, that puts more stress on the tendons and ligaments rather than the muscle itself

Do try to minimize how much you bend the support leg. That increases the stretch reflex, and ultimately your flexibility. It also allows you to snap back more quickly.

In the video you linked to, the leg was bent too much to be of much use on the hamstring and glutes. Instead, it will be more focused on the hips. It also loses the stretch reflex making it less bio-mechanically advantageous.

Thanks for your answer. I'm not sure whether I've understood the flexibility part correctly: Personally I have more flexibility issues with the stiffed-legged version than when I do it like this: youtube.com/watch?v=LA1n11ph_wU
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zero-divisorApr 25 '13 at 20:36

But the way it is performed in the video I linked to is the more similar to the bilateral deadlift, isn't it? Why is the bilateral deadlift performed mostly with non-stiff legs if this is less advantageous?
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zero-divisorApr 26 '13 at 5:05

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@zero-divisor, take some time to understand what the movements are supposed to be addressing. The conventional deadlift is a full body lift, meant to be loaded very heavy. You need the stability to lift in excess of 300lbs. Single leg and stiff leg (two legs planted) are more isolation exercises targeting a specific group of muscles.
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Berin LoritschApr 26 '13 at 11:40

The paper compares Squat, Leg Curl and stiff-legged (2-legged) Deadlift, but this does not correspond to my question, does it?
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zero-divisorApr 25 '13 at 20:33

@zero-divisor You are correct regarding the paper. The paper is to prove the muscle recruitment when doing stiff-legged deadlifts. With this evidence, I thought I also gave you my reasoning for the "single leg exercises to include single leg stiff-legged deadlift. Please let me know if I'm not understanding your questions.
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DrTrungNguyenApr 25 '13 at 20:36

I'm not questioning that the stiff-legged deadlift is a good exercise. I'm just wondering why the single leg deadlift is mostly performed with stiff legs.
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zero-divisorApr 25 '13 at 20:38